31 S Dr Daubeny on the Ancient Volcanoes of Auvergne. 
whilst the summit and the base alike consist of trachyte. It 
dips gradually to the east ; so that about half a league from 
Teyzac, on the road to Murat, it reaches the level of the road. 
Different as the tuff appears from the trachyte which it accom- 
panies, it will be found on examination, that the fragments of 
which it consists are cemented always by a basis of the latter 
rock, and that a passage from the one to the other proceeds by 
imperceptible gradations. The fragments are in general a tra- 
chyte of a more compact character than the paste which cements 
it, but we also find basalt and cellular lava intermixed ; and I 
- remarked beds or veins of the same description of stone so much 
resembling tripoli, which I have already noticed as occurring at 
Mont d’Or. A little beyond Teyzac, near Vic en Carladez, I 
saw an instance of an incurvation of its strata, like what happens 
in the zones of an agate ; the layers of this rock being contorted 
uniformly one to the other in the middle of the body of the tuff, 
in a manner which reminded me of the natural arch of clay- 
porphyry which I observed in Arran, and which may now be 
seen figured in the Plates to Dr MacCulloch’s work on the 
Western Islands ; and of a similar conformation of the sandstone 
at a village near Edinburgh. 
It is curious that the tuff* sometimes contains perfectly isola- 
ted portions of a quartzy sandstone, which seem to bear no ana- 
logy whatever to the other constituents of the rock ; and at Sa- 
lers, where the fragments are in general so minute, that the 
whole has much the appearance of a ferruginous sandstone, im- 
pressions of leaves and branches of trees, often in no respect^ mi- 
neralized, but reduced to an impalpable powder, by the ordina- 
ry process of decay, are found contained in it. In other cases, 
where the tree has wholly decayed, the hollow which it occupied 
in the midst of the tuff still remains to indicate its having exist- 
ed. These circumstances tend, in a still greater degree, to 
identify the Trachyte Formation of Auvergne with the ordina- 
ry products of mud volcanoes. 
And here I may conclude what I have to say of the volcanic 
rocks found in Auvergne, a country which deserves to be studied 
by all who are anxious to arrive at an unbiassed decision respect- 
ing the origin of a class of rocks which occupy no inconsider- 
able portion of the earth’s surface. We are here enabled, as it 
